This pilot study will use dextromethorphan as a phenotypic probe of cytochrome P450 2D6 (CYP2D6, debrisoquin hydroxylase) activity to identify the functional metabolic phenotype (extensive vs. poor metabolizer) of heart failure patients receiving chronic therapy with carvedilol. Specifically, we are interested in determining the frequency of the poor metabolizer (PM) phenotype in this select group of patients.